The present invention is directed to video signal treatment, and is especially directed to treatment of color video signals to achieve improved color separation in displays or images created using the signals.
The composite video signals often used in terrestrial and cable broadcast systems consist of three video components mixed together. A luminance component (LUMA) is at baseband and has a signal width of up to about 6 MHz (MegaHertz). Color components (U and V; CHROMA) are quadrature modulated with a subcarrier signal. U.S. standards establish the color component signals as modulated with a 3.58 MHz subcarrier. European standards (PAL) establish the color component signals modulated with a 4.43 MHz subcarrier.
Because the component signals' bandwidths overlap, crosstalk can occur when separating the component signals to their original luminance and color contributions to an image. Some solutions employ comb filters to reduce such crosstalk by vertically filtering the signal. Employing such vertical filtering to succeeding signals representing adjacent lines in an image effects temporal or time-related filtering. Such temporal filtering addresses like portions in an image appearing along a common axis in the image, generally perpendicular to the parallel lines established by the succeeding signals. Modern adaptive digital comb filters can achieve good crosstalk rejection while introducing a minimal amount of filter artifacts.
A problem occurs when comb filters operate only on the signal frequencies near the color subcarrier. Such an employment of filtering seems like a good solution because LUMA-CHROMA crosstalk can only exist in the region inside the CHROMA bandwidth. That band width is specified by standards to be about 1.3 MHz. The filters used to limit bandwidth (low pass filters limit the baseband; bandpass filters limit the composite signals) are not ideal. That is, the filters have losses as the signals approach the stopband. The losses typically amount to about 33% loss at 1.3 MHz. The portion of CHROMA that falls outside the comb filter band of operation becomes false LUMA, which is manifested in the resulting image as “dot crawl”. Dot crawl is a moving pattern of dots on all horizontal color transitions.
One could design a comb filter designed to operate on the entire video frequency spectrum to eliminate dot crawl completely for horizontal color transitions. However, such a filter would cause increased artifacts on vertical LUMA transitions.
There is a need for a method for generating a representation of a particular signal among a plurality of signals representing lines in a display that avoids introducing artifacts or other manifestations of signal interference.